Pull a valve cover and see if there is any mud under it. That is a good indication that a PO did not change the oil frequently. At the same time, you can run the engine and see if you have any "lazy" rocker arms - indicating a worn cam.
If the head under the R cover is clean, that is a good sign. If it is muddy, not a good sign...
If the engine has a factory original cam, and the engine now has noisy lifters, this usually indicates a worn cam, or worn rocker arm pivot balls. All this will have to be determined by removing the valve covers.
I had a 71 Pontiac Lemas convert in my shop - once upon a time - that had a mild skip, noisy rockers and was low on power. After the usual checks, I pulled a valve cover and found some SERIOUS mud. It also had a few rockers that barely moved.
I knew the owner personally and asked her if she changed her oil recently...
You know what the answer was?
"Oh, you have to change the oil?"
Ok, I pulled the intake off and could hardly see the lifters, or for that matter - the pushrods. It was the worst I had ever seen. I scraped out a SHOEBOX of mud. The cam had 2 1/2 round lobes!
I cleaned everything and installed a good used cam from a Pontiac 421, Cloyes roller chain set, I also had to replace 2 rocker assemblies. To my amazement, the engine ran like a freight train. (350 2 barrel Pontiac enigne)
Even she was surprised.
Moral of story? There are a few of them.
1. Change yer damn oil!
2. A muddy engine isn't dead yet!
3. Noisy lifters = worn parts
4. Fresh cam = POWER
Sorry for the book, but maybe it helps explain stuff.